1. Field of the Invention
The present invention relates to a power transfer circuit. More specifically, it relates to a power transfer circuit which is applied to a television receiver or a radio receiver employing a power circuit receiving DC voltages from AC power sources through a rectifier circuit, to perform switching operation for stabilizing the derived DC voltage when the AC power sources are different in voltage level from each other.
2. Description of the Prior Art
In general, AC power sources, employed in various countries may vary in voltage levels utilized. Specifically there are areas employing AC power sources of both 110 V and 220 V, of only 110 V and of only 220 V. When television receivers and the like are used in such areas employing AC power sources of different voltage levels, it is required to obtain predetermined constant DC voltages from such different supply voltages. Therefore, television receivers and the like generally contain power transfer circuits.
FIG. 1 is an electric circuit diagram showing a conventional power transfer circuit. In FIG. 1, an AC power source 1 is connected to both ends of a tap-down type power transformer 2, one end of which is connected with one switching terminal 6 of a changeover switch 4 while a center tap thereof is connected to the other switching terminal 7. A bridge rectifier circuit 8 formed by rectifying diodes D1, D2, D3 and D4 is connected between a fixed terminal 5 of the changeover switch 4 and the other end of the power transformer 2. The junction between the rectifying diodes D1 and D2 is grounded while a load 9 and a smoothing capacitor C1 are connected between the junction of the rectifying diodes D3 and D4 and ground.
The conventional power transfer circuit, having a structure as hereinabove described, when the circuit is applied to an AC power source of 110 V, the said one switching terminal 6 is connected to the fixed terminal 5 of the changeover switch 4 so that an AC voltage of 110 V is rectified by the bridge rectifier circuit 8 to derive a DC voltage E.sub.0 of 110 V. When, on the other hand, the circuit is applied to an AC power source of 220 V, the changeover switch 4 is so switched as to connect the other switching terminal 7 to the fixed terminal 5, whereby an AC voltage of 220 V is stepped down to 110 V by the power transformer 2 since the other switching terminal 7 is connected to the center tap of the power transformer 2. The stepped-down voltage of 110 V is rectified by the bridge rectifier circuit 8, whereby a DC voltage E of 110 V is derived.
The changeover switch 4 is thus switched in the aforementioned manner, whereby a constant DC voltage is outputted from the bridge rectifier circuit 8 whether the voltage at the AC power source is 110 V or 220 V.
In the conventional power transfer circuit as shown in FIG. 1, the AC voltage is stepped down by the power transformer 2 to be supplied to the bridge rectifier circuit 8, whereby the demand from the power transformer 2 is increased. Further, in order to separate the internal chassis in a television receiver, a separate transformer must be newly provided at the input side of the power transformer 2, leading to an increase in the number of elements.